Windows and glass in public transportation vehicles such as buses or subway trains can be subjected to a tremendous amount of abuse. The windows can be damaged by both incidental scratching during cleaning or they can be maliciously damaged by vandalism. Vandals damage the windows by scratching or abrading the surface of the window with items such as lava rock, diamonds, abrasive papers or abrasive cloths. Vandals can also damage the window by painting or coloring the surface of the window. Cleaning processes have been defined to eliminate damage by painting or coloring. However, scratching of glass by vandals presents a significant problem. In one major city for example, approximately 40 percent of the bus windows have been vandalized by scratching and close to 80 percent of the subway train windows. The public transportation officials now call this type of graffiti "scratchiti". The best approach to stopping graffiti historically has been is to remove the graffiti immediately from the damaged area. This graffiti prevention system which is known as "zero tolerance", has been extremely successful in eliminating the written and painted vandalism. The scratched glass however is extremely difficult and expensive to repair and as a result, the zero tolerance approach to scratchiti prevention and elimination is cost prohibitive. The vandalism issue tarnishes the overall image of an entire city's transportation system. The vandalized glass leads to lower ridership because of the reduced perception of safety on the vehicle by the public. The vandalism ultimately leads to lost revenue for the public transportation system and substantially high repair costs. A cost effective method of restoring the window to its original appearance is needed.
Several approaches have been evaluated to restore the windows to its original appearance. The first approach has been to repair the glass by a multi-step abrasion/polishing method to remove the scrachiti. The abrasion steps remove glass to the depth of the scratch with diamond abrasives and then with subsequently finer grades of diamond or aluminum oxide abrasives the surface of the glass is polished to its original appearance. The abrasive materials are expensive and the time required to completely abrade and polish the surface of the glass can be 6-8 hours depending on the depth of the damaged areas.
The second approach commonly used to repair and protect windows from scratches is to coat the damaged window with an epoxy coating (Enhancement of Vehicle Glazing for Vandal Resistance and Durability by Daniel R. Bowman, Mar. 25, 1996, available from the United States Transportation Research Board). The damaged window is typically first scrubbed clean before coating with an epoxy coating. The epoxy coating is used to fill the defects on the windows and restore the window to a state of clarity where signs can be read through the window. To apply the coating, the window must be removed from the vehicle and the window must be cleaned and primed. The coatings are applied and cured in a clean environment. The coatings currently available however are easily scratched by the same method used to scratch the glass. Once the coating is damaged, it is difficult to apply a subsequent coating due to poor adhesion of the coating to the first layer. The process to replace the damaged coating with a new coating is time consuming and expensive.
U.S. Pat. No. 4,301,193 disclosed a method of removing scratches and impregnated dirt from transparent plastic article by applying a polishing formulation containing a mild abrasive to a surface to be treated, polishing the surface with a soft material and applying to the polished surface a liquid silicone formulation and wiping the surface with dry soft material.
U.S. Pat. No. 5,194,293 disclosed a method for restoring weathered plastic surfaces and for enhancing resistance of the treated surface to sunlight by cleaning the surface, polishing with a mild abrasive, applying a first layer of an uncured adhesive material that is ultraviolet curable and applying an outer coat of a compatible curable adhesive over the uncured to form a semi-permanent outer surface on the article. Curing is initiated by exposure to ultraviolet radiation.
GB 2310862 disclosed a resin formulation for repairing a transparent glass member e.g. an automobile windscreen. The fluid resin is injected into a crack, which mixes with the trapped air in the damaged area. A device withdraws the fluid resin and trapped air, separates resin and trapped air and reinject separated resin in the cracked area. Repeated injections are possible because the resin has a select viscosity.
U.S. Pat. No. 5,512,116 disclosed a method of repairing an automobile windscreen by inserting a first resin of selected viscosity into the surface portions of the crack and then inserting a second resin of selected viscosity which is higher than the first into the remainder of the crack.